1. Field of the Invention
The present invention relates to an autofocusing apparatus suitable for a sighting telescope incorporated in a surveying instrument such as an automatic level, a transit, a theodolite, etc.
2. Description of the Related Art
A conventional surveying instrument such as an automatic level (auto-level), a transit, or a theodolite, etc., is generally provided with a sighting telescope, a level and a measuring device which measures a rotational angle, a descending angle, and a ascending angle, etc. When this type of surveying instrument is used, the sighting telescope is positioned horizontally, and subsequently the horizontal and vertical adjustments for the sighting telescope are performed. Thereafter, the sighting telescope is aimed at a reference object or a reference point so that the reference object is sighted by a surveyor.
For instance, the optical system of the sighting telescope of an automatic level includes an objective lens group, a focusing lens group and an eyepiece, arranged in that order from the object side. The position of the focusing lens group is adjusted depending on the object distance, so as to form a sharp object image on a reticle provided on the focal plane. The object image formed on the reticle can be viewed through the eyepiece.
In a conventional sighting telescope provided with an autofocusing system, immediately after the AF start button of the autofocusing system is depressed by the user, the focusing lens group of the system is driven from a current position (e.g., initial position) to another position to bring a sighting object into focus. According to a phase-difference detection AF system, a focal point which is firstly detected by the AF system is regarded as an actual focal point for the sighting object, so that the AF system drives the focusing lens group to an axial position thereof which corresponds to the initially detected focal point to stop the focusing lens group thereat.
A sighting telescope is different from a telephoto-photographic lens used for cameras or regular observational telescopes; namely, that the focus detection range of sighting telescope from an infinite distance to the close-up extremity is much greater (e.g., by a number of centimeters). When such an excess detection range is converted into the amount of movement of the focal point, the distance of movement exceeds from 100 mm through 200 mm. The focus detection range of an AF unit of a phase-difference detection type is approximately .+-.15 mm from an assuming focal point, so that the focus detection range of the AF unit can cover merely a fraction of the whole focus detection range of a sighting telescope. Accordingly, the phase-difference detection AF unit is used for a sighting telescope, wherein there is a high probability that an actual focal point (target focal point) of the sighting object be positioned out of the focus detection range. Therefore, specifically in the case of the conventional sighting telescope being provided with an AF system having a phase-difference detection AF unit, there is a probability that other objects (non-target objects) rather than the sighting object (target object) be brought into focus if the AF system moves the focusing lens to an axial position thereof which corresponds to the initial detected focal point.